Heparan sulfate (HS) is an essential glycan that is present in large quantities on the cell surface and in the extracellular matrix. HS participates ina variety of physiological and pathophysiological functions, including blood coagulation, inflammatory response and embrynic development. HS is a highly sulfated polysaccharide. Heparin, a special form of HS, is a commonly used anticoagulant drug. The uniquely distributed sulfation pattern of HS is believed to regulate its functional specificity. The wide range of biological functions of HS attracts considerable interest in understanding the mechanism for controlling the biosynthesis of HS. HS biosynthesis involves a series of specialized HS sulfotransferases, glycosyltransferases and an epimerase. Using HS biosynthetic enzymes, our labs can produce an array of HS with unique sulfation patterns and functions. Our success has proved the feasibility for studying the control of the biosynthesis of specific sulfated saccharide sequences. Our hypothesis is that the HS biosynthesis is controlled by a series of up- stream modification steps involving N-sulfation and C5-epimerization with or without concurrent 2-O-sulfation. In this proposal, we will investigate the mechanism used by C5-epimerase and 2-O-sulfotransferase to control the synthesis of specific saccharide sequences that exhibit the anticoagulant activity and the activity in stimulating growth factors and growth factor receptor mediated cell proliferation. We will use structurally defined oligosaccharide substrates with precisely controlled N-sulfation patterns to prove the substrate specificity of C5-epimerase and 2-O-sulfotransferase. Finally, we plan to prove a unique biosynthetic pathway for preparing a highly sulfated domain that is commonly found in heparin. The success of this project will lead a comprehensive new approach to investigate the biosynthesis of HS, potentially leading to the development of novel HS-based therapeutics.